Code Division Multiple Access (CDMA) is a multiple access technique in which data symbols are spread over orthogonal code sequences or near orthogonal code sequences, or a combination of both. Conventional CDMA encoding is a two-step process in which a binary code is mapped to a Quadrature Amplitude Modulation (QAM) symbol before a spreading sequence is applied. Although conventional CDMA encoding can provide relatively high coding rates, new techniques/mechanisms for achieving even higher coding rates are needed to meet the ever-growing demands of next-generation wireless networks. Low Density Spreading (LDS) is a form of CDMA used for multiplexing different layers of data. LDS uses repetitions of the same symbol on specific layers at nonzero locations in a time/frequency resource. As an example, in LDS-Orthogonal Frequency Division Multiplexing (OFDM) a constellation point is repeated over nonzero frequency tones of a LDS block. In Sparse Code Multiple Access (SCMA), a multidimensional codebook is used to spread data over tones without necessarily repeating symbols.
The decoding complexity of SCMA may become prohibitive for very high rates, resulting from, for example, a large number of layers, very large constellations, or a combination of the two.